Merge request stuck at "Merging"

Problem to solve

This merge request is stuck at “merging”.

We’re using marge bot. The marge-bot logs report that an error was received from marge-bot when it attempted to merge the MR through the GitLab API:

Now the MR cannot be closed, nor deleted. The interface shows a “Re-open MR” button but it has no effect. The GitLab API reports that the state of the MR is locked.

Steps to reproduce

I’m not sure how to reproduce this since I’m not sure how it happened.

Configuration

We require 2 approvals. We use semi-linear history.

Versions

We’re using "GitLab.com SaaS "